Mystique Network Upgrade Adopts London Subset

Ethereum Classic activated the Mystique network upgrade at block 14,525,000 on February 12, 2022. Mystique adopted a subset of Ethereum's London hard fork changes, specified in ECIP-1104.

## Included EIPs

- **EIP-3529 — Alternative Refund Reduction**: Eliminated SELFDESTRUCT refund (24,000 → 0 gas) and capped SSTORE refunds at 20% of transaction gas. This change made gas token contracts (CHI, GST2) economically unviable, removing a long-standing abuse vector that complicated gas estimation.
- **EIP-3541 — Reject New Contracts Starting with 0xEF**: New contract deployments with bytecode starting at 0xEF are rejected. Existing contracts are unaffected. This reserves the 0xEF prefix for the future EVM Object Format (EOF), preventing bytecode collisions.

## Omitted EIPs

- **EIP-1559 (Fee Market Change)**: EIP-1559 burns the transaction basefee, permanently destroying tokens. This conflicts with ETC's fixed monetary policy under ECIP-1017 (~210.7M cap). The community chose to defer EIP-1559 adoption — the Olympia network upgrade redirects the basefee to a community treasury (ECIP-1112) instead of burning it, preserving the monetary policy while adopting the fee market mechanism.
- **EIP-3198 (BASEFEE Opcode)**: Depends on EIP-1559; without an active basefee, the opcode has no defined return value.
- **EIP-3228 (Difficulty Bomb Delay)**: ETC permanently removed the difficulty bomb in ECIP-1041 (block 5,900,000). Not applicable.

## Client Requirements

- Core-Geth v1.12.9 or later
- Hyperledger Besu with ETC Mystique support